1. Field of the Invention
The present invention relates to an exhaust system for a motorcycle.
2. Description of the Related Art
Many of exhaust systems for motorcycles, each of which has a multicylinder engine mounted thereon, are so constructed that exhaust pipes, the number of which is equal to the number of cylinders, extending from exhaust ports of the engine are collected and integrated into one collecting pipe or portion, and an expansion chamber is connected to a downstream side thereof.
The expansion chamber has a large weight, and in a case where the expansion chamber is arranged beside or above a rear wheel as seen in an exhaust system of a general motorcycle, the expansion chamber is separated from a gravity center of the vehicle body, causing the weight to be distributed and causing the position of the gravity center of the vehicle body to be raised, which causes a fear of lowering the driving stability
As disclosed in Japanese Patent Application Laid-Open No. HEI 4-266579 (Prior Art 1), Japanese Patent No. 2875267 (Prior Art 2), and Japanese Patent Application Laid-Open Publication No. HEI 8-6597 (Prior Art 3), there has been proposed an exhaust system for a motorcycle having an expansion chamber arranged under an engine. In these exhaust systems, an exhaust throttle valve for controlling a flow rate of exhaust gas, and an exhaust gas sensor (generally, an O2 sensor) for detecting an exhaust gas-component are provided in the vicinity of the expansion chamber.
In the exhaust system of the Prior Art 1, the exhaust gas sensor is arranged inside the expansion chamber disposed just under the engine. Moreover, in the exhaust systems of the Prior Arts 2 and 3, the exhaust throttle valve is disposed inside the expansion chamber.
However, in a case where the exhaust gas sensor is disposed inside the expansion chamber as the exhaust system of the Prior Art 1, water vapor contained in the exhaust gas is condensed into water in the expansion chamber and hence adhered to the exhaust gas sensor when the engine is cold, which causes a problem of prohibiting the exhaust gas sensor from operating until the condensed water is vaporized along with warming up of the engine.
Further, in a case where the exhaust throttle valve is disposed inside the expansion chamber as the exhaust system of the Prior Arts 2 and 3, the exhaust throttle valve must be large, thereby causing the exhaust system to also become large-sized. Furthermore, since the expansion chamber has a large flow passage area, the exhaust pulsation weakens, and hence, a sufficient output increasing effect cannot be expected.